The present invention relates generally to RFID tags, and more particularly to RFID tags to record temperature or other environmental conditions.
Active RFID tags are well known today. For example, the Matrics Group manufactures “MicroSensys™” Active RFID tags, and KSW Microtec manufactures “Smart Active Label™” Active RFID tags. An Active RFID tag includes a small battery, and transmits RF signals via an integral antenna. The battery adds significant cost to the RFID tag. The RFID may be preprogrammed with an identity that represents the identity of the product which bears the RFID tag. The transmitted RF signals typically include the identity of the Active RFID tag. If the Active RFID tag includes data such as a recorded temperature, the Active RFID tag may transmit the data as well. An Active RFID tags can effectively transmit to a receiver up to 100 meters away.
Passive RFID tags are also well known today. A Passive RFID tag does not include a battery; instead, the Passive RFID tag has an integral antenna which receives RF signals from an Active RFID tag or remote transceiver. The transmitted RF signals power the Passive RFID tag, i.e. the Passive RFID tag captures the energy of the RF signals which it receives and uses the energy to drive transceiver and other circuitry on the Passive RFID tag. The transceiver circuitry on the Passive RFID tag transmits the identity coded into the Passive RFID to identify the product which bears the Passive RFID tag. The Passive RFID tag may also include a memory to store any type of information transmitted by the Active RFID tag or other remote transceiver. Typically, Passive RFID tags have a range of about three meters.
The known remote transceiver reads the identity information broadcast by the Passive RFIDs or Active RFIDs. The remote transceiver can read RFID tags on packages contained in a box to determine what packages the box contains or to confirm that the packages are all there. Also, the remote readers can read RFID tags on packages at a cash register to determine an amount to charge to the customer (as does a known bar code reader). Also, the remote readers can detect packages at a store exit which bear RFID tags which have not been “disabled” at a cash register, and thereby detect shop lifters.
It was also known for an Active or Passive RFID tag to include a temperature, humidity or other environmental sensor. For example, the Matrics Group MicroSensys™ Active RFID tags and KSW Microtec Smart Active Label™ Active RFID tags include temperature sensors. The temperature sensor measures the ambient temperature, and circuitry on the RFID tag containing the sensor records information about the temperature, such as whether the ambient temperature rose above a predetermined upper limit or fell below a predetermined lower limit. (In the case of the Passive RFID tag containing the temperature sensor, the Passive RFID tag can only monitor and store the temperature when the Passive RFID tag is energized by an external RF signal.) For example, if the RFID tag is attached to a temperature sensitive product such as a food package, the RFID tag will record whether the food was exposed to excessively hot or cold temperatures during shipment. The RFID tag may also periodically record temperature samples over time. If the RFID tag stores a single indication whether the temperature went outside a tolerable range, the RFID tag needs only a “Read Once Memory”. If the RFID tag stores periodic temperatures, then the RFID tag needs a more expensive “Write Many Memory”. Maintaining the product within a specified temperature range during shipment may be a condition to pay the shipper, and for a customer to accept the product. While such “composite” RFID tags are important, they tend to be costly due to the requirement for the temperature sensor, especially when there are many packages to monitor.
An object of the present invention is to record temperature or other environmental condition on an RFID tag in a cost effective manner.